In association with the widespread use of image-forming apparatuses such as a copying machine for electrophotography and a printer, the apparatuses have been finding use in an expanded variety of applications, and quality requested of an image formed by each of the apparatuses has become more and more severe in recent years. For example, each of the apparatuses has started to be used as not only a copying machine for paperwork for copying an original manuscript but also a digital printer as an output for a computer, an apparatus for copying a high-definition image such as a graphic design, or an apparatus for a light printing application where additionally high reliability is requested. Color tone reproducibility at not only a solid portion but also a halftone site is strongly requested in such application where high-definition image quality is strongly requested.
When an image is formed with magnetic toner using magnetic iron oxide particles each having a deteriorated degree of blackness in a system using magnetic toner, the color tone reproducibility of the image may involve, for example, the following problem: a black portion becomes reddish, so a desired tinge cannot be reproduced, and those who view the image feel abnormal. In addition, the use of magnetic iron oxide particles different from each other in degree of blackness causes a reduction in quality of an image formed with the particles due to image density unevenness.
An additional improvement of the system using the magnetic toner for copying a high-definition image or for a light printing application has been requested because those problems become remarkable particularly at a halftone site with a small toner laid-on level.
In addition, at the same time, the system has been strongly requested to provide images each having high sharpness stably over a long time period. Accordingly, wide-latitude images each of which: has good developing ability; and is free of fogging must be stably formed, and performance requested of the magnetic toner has become more and more sophisticated.
The quality of an image to be formed by a magnetic one-component developing method largely depends on the performance of magnetic toner to be used. Fine powdered magnetic iron oxide particles, which are mixed and dispersed in the magnetic toner particles of the magnetic toner in a considerable amount, each act as a pigment, and, at the same time, each affect the charging characteristic of the magnetic toner. That is, the degree of blackness of the magnetic toner is determined by the degree of blackness of each of the magnetic iron oxide particles, and each of the magnetic iron oxide particles acts as a leak point, so the electrical characteristics and dispersing ability of the magnetic iron oxide particles affect the electrical characteristics of the magnetic toner. As a result, the magnetic iron oxide particles affect the developing characteristic and durability of the magnetic toner. Accordingly, a large number of proposals have been conventionally made on magnetic iron oxide particles to be incorporated into magnetic toner particles.
The degree of blackness of a magnetic iron oxide particle, in particular, a magnetic iron oxide particle containing Fe(2+) such as a magnetite particle depends on the state of presence of Fe(2+). However, Fe(2+) in the magnetic iron oxide particle is apt to be oxidized, so the oxidation advances as time elapses after the production of the particle, and hence the content of Fe(2+) reduces. As a result, the degree of blackness of the particle reduces, the particle becomes strongly reddish, and the charging characteristic of the particle changes.
It should be noted that the expression “Fe(2+) of a magnetic iron oxide particle” as used in the present invention refers to a divalent iron atom, and comprehends an iron atom present as FeO or Fe2+. In addition, the expression “Fe(3+)” refers to a trivalent iron atom.
Patent Documents 1 to 3 each disclose a technique involving increasing the amount of Fe(2+) of a black component with respect to the entirety of magnetic iron oxide particles in order that magnetic iron oxide particles each of which: has a high degree of blackness; and is excellent in environmental resistance may be obtained. Such magnetic iron oxide particles, if used, each show a somewhat high degree of blackness immediately after the production of the particles. However, the particles are each vulnerable to deterioration over time and poor in stability because each of the magnetic iron oxide particles is subjected to a reduction treatment, has a fine structure on its surface, or is oxygen-deficient. Accordingly, the particles are not preferable in an application where a document is stored for a long time period such as light printing because the degree of blackness of each of the particles is apt to deteriorate.
A technique involving adding various elements to magnetic iron oxide particles has also been disclosed. Patent Documents 4 and 5 each describe a magnetic iron oxide particle having a composite iron oxide coat layer containing Co, and Patent Document 6 describes a magnetic iron oxide particle having a composite iron oxide coat layer containing Zn. In addition, Patent Document 7 describes a magnetic iron oxide particle containing composite iron oxide containing an element such as Mn, Zn, Cu, Ni, Co, or Mg, Patent Document 8 describes a magnetic iron oxide particle having an Si-containing coat layer, and Patent Document 9 describes a Ti-containing magnetic iron oxide particle.
Each of those added elements plays the following role in suppressing the deterioration of the degree of blackness of each of the magnetic iron oxide particles: a particle is coated with an added element in order that Fe(2+) may be out of direct contact with an external atmosphere, or Fe(2+) is substituted by an added element the degree of blackness of which does not reduce.
The degree of blackness of each of magnetic iron oxide particles obtained by such method is prevented from reducing to a certain extent, and the deterioration of each of the particles over time is suppressed to a certain extent. However, a degree of blackness is apt to vary from magnetic iron oxide particle to magnetic iron oxide particle, and an image formed with the particles is apt to show density unevenness. In addition, a degree of blackness at a halftone site of the image must be additionally improved.
A method of improving the developing ability of magnetic toner by controlling the dispersing ability of magnetic iron oxide particles in the magnetic toner particles of the toner has also been proposed. The following proposal has also been made: the dispersing ability of a magnetic iron oxide particle is controlled by specifying the dielectric loss tangent of magnetic toner.
A disclosed in Patent Document 10 is as described below. The dielectric characteristics of magnetic toner are controlled by using magnetic iron oxide particles each having the following features: each of the magnetic iron oxide particles contains silicon in itself, and the surface of each of the magnetic iron oxide particles is coated with iron oxide composite containing silicon and zinc. However, the technique is susceptible to improvement in order that an image having an improved degree of blackness at a halftone site, good developing ability, and suppressed fogging may be obtained.    Patent Document 1: JP 2992907 B    Patent Document 2: JP 3239220 B    Patent Document 3: JP 2001-002426 A    Patent Document 4: JP 6-100317 A    Patent Document 5: JP 8-133744 A    Patent Document 6: JP 8-133745 A    Patent Document 7: JP 4-162050 A    Patent Document 8: JP 2006-133735 A    Patent Document 9: JP 2003-162089 A    Patent Document 10: JP 2003-195560 A